1. Field of the Invention
The present invention relates generally to the art of fuel burners and more particularly to the art of burners for solid particulate fuel.
2. Description of the Prior Art
The prior art contains numerous examples of burners designed to burn a wide variety of liquid, gaseous and solid fuel materials. Since the present invention relates primarily to the combustion of bio-mass fuels, the description of the prior art will be addressed primarily to combustors for bio-mass fuels. Furthermore, for reasons which will soon become apparent, the description will be more specifically addressed to wood combustors.
For many years, researchers have been working on devices for efficiently burning bio-mass materials. The reasons are quite obvious, especially in these times of rising fuel prices and energy shortages. Wood is a renewable resource and is readily available in most geographic areas. Furthermore, scrap wood is available for energy use. Other bio-mass materials are also available in commercially significant quantities.
While the combustion of bio-mass materials has been known since the early days of mankind, the burning of wood in a controlled manner has received less attention from the scientific community than has been the case with the burning of other fuels such as gas, oil or coal. In reality, the burning of wood remains somewhat of an art as opposed to a developed science. To understand the reason for the failures of past inventive efforts, it is necessary to understand the factors which influence the efficient combustion of such solid fuels.
Two primary factors are particle size and moisture content. Particle size is important because small particles expose more total surface than larger particles and the burning rate depends on exposed surface area. Particle volume or mass increases as the cube of its diameter, while surface area increases only as the square of the diameter. Burning time is, therefore, directly proportional to particle size. Moisture is important because the wetter the wood, the more difficult it is to burn. These basics then dictate that a large number of small particles of dry wood be used in any efficient wood burning system.
Another primary factor relates to the controls necessary to regulate the combustion process, i.e., controls affecting combustion efficiency, emissions, ash condition, etc. Other related factors include the reactivity of the fuel with the oxidizer, burning temperature, residence time in the combustion zone, and the intimacy of mixing of the fuel and oxidizer.
Several types of suspension firing systems are known for use with wood and other solid materials. Suspension systems are those in which the fuel is supported by air and the burning gases, and there is no bed of fuel supported by a grate, hearth or retort. Burning particles are suspended in the flame until consumed or extinguished. Laminar or axial flow flame burners are known, and these typically include a system for injecting combustion air and fuel in parallel streams. Laminar or axial flow burners are commonly used in large boilers, usually with several individual burners firing in a common furnace. Such burners require a comparatively large furnace and the quality of combustion depends upon the manner in which the burners are arranged.
Furnaces fired by a single suspension burner generally use some form of cyclonic or vortex combustion system where fuel is injected tangentially into a cylindrical or conical combustor and the burning particulate material revolves about the axis of the flame. Cyclonic furnaces provide long flow paths for the burning of fuel, thus creating longer residence time within a smaller combustion zone. Boilers having small combustion chambers are generally limited to liquid and gas fuel firing, and if wood is the chosen fuel, the combustion system must be very efficient.
Cyclonic combustors may be either of two basic types, i.e., single vortex or double vortex. In single vortex systems, the fuel enters one end of the combustor and hot products of combustion exit the other. In a double vortex combustor, some or all of the fuel is injected tangentially near the end from which the flame exits. Burning progresses in two concentric rotating streams. While the present invention is not to be limited to any particular type of vortex, and in fact can be used with double vortex combustors, reference will be had here and in the remaining description to a single vortex type.
The system of the present invention is most suitable for use with dry fuels as pointed out above. Commonly, in wood burning, "dry" refers to wood having a moisture content of less than 12%. Moreover, it is desirable to have a highly reactive fuel, i.e., wood or wood char having a relatively high oxygen content. Oxygen contents between 35-45% are most advantageous. It is also important in burning wood to have a relatively low ash content, for example, less than 6% and a relatively high ash fusion temperature, for example, over about 2200.degree. F. The combination of fuel properties mentioned above may vary, and none of these factors is deemed to be limiting as to the scope of the invention.
It is also known that the combustion air may be introduced in a variety of ways and, in some instances, in multiple stages for minimizing the generation of nitrogen oxides. For example, one system for burning wood is disclosed in U.S. Pat. No. 3,856,455 issued on Dec. 24, 1974 to Otway et al. for "Method and Apparatus For Mixing and Turbulating Particulate Fuel With Air For Subsequent Combustion." In this patent, particulate fuel is mixed with a relatively small quantity of air and supplied into one end of a chamber so as to promote turbulence in the mixture as it passes along the chamber to an outlet. Ignition takes place at the outlet and further air is added to the mixture in the region of the outlet to permit full combustion of the entrained fuel when discharged therefrom. The turbulation is maximized in that the air and fuel inlets are positioned to provide a cyclonic movement. Suitable guides may be provided within the chamber to cause further turbulent effect. This patent then provides for the introduction of primary and secondary air and indicates that the amount of air should be selected so that the primary and secondary air, when combined, provide sufficient air to support combustion of the suspended fuel.
Another device for burning wood fuel is disclosed in U.S. Pat. No. 4,249,471 issued on Feb. 10, 1981 to Gunnerman and entitled "Method and Apparatus For Burning Pelletized Organic Fiberous Fuel." In this patent, a pellet of solid fuel is mixed with a flammable gas or liquid and burned in a combustor. The combustor contains an overfire system which is designed for efficient combustion and the production of a minimum number of solid combustible products as ash. The flammable gas mixed with the solid fuel material may be made by burning pellets of an organic fiberous material. The wood particles are injected into the flammable gas and, together with air, and are introduced tangentially to provide a cyclonic movement of the solid particles about the burning flammable gas stream which passes axially through the combustor inlet. A diffuser is also provided to insure proper combustion of the particles and gas as they progress through a confinement cylinder.
While introduction of combustion air in multiple stages is recognized, the amount of air introduced and the location of air introduction has still not been optimized in theory or practice.